Determination and differentiation of the skeletal muscle lineage is controlled by the MyoD family of myogenic transcription factors. These factors, along with the MEF-2 family of transcription factors, auto-activate and cross-activate the expression of each other, resulting in an autoregulatory, positive feedback network that maintains the myogenic phenotype. Determination and differentiation of cells in this lineage also require cell-cell contact between muscle precursors. However, the identities of the cell surface proteins involved in this phenomenon, and their specific roles in myogenesis, are not well understood. Sialomucins are cell surface proteins characterized by mucin domains, which are proline-, serine-, and threonine-rich regions on which a high percentage of the serine and threonine residues are O-glycosylated. These O-linked glycans are central to sialomucin function, as specific sialomucins are known to serve as high affinity ligands for selectins; selectins are transmembrane receptors that recognize sialylated carbohydrate structures on these sialomucins during leukocyte trafficking. The biological roles of sialomucins outside of leukocyte trafficking are poorly understood. We have identified Cd164 as a gene expressed in proliferating C2C12 myoblasts that is specifically upregulated during differentiation. Cd164 encodes a widely expressed cell surface sialomucin that has been implicated in regulation of cell proliferation, differentiation and adhesion of hematopoietic precursors. Stable overexpression of Cd164 enhances myoblast differentiation. Treatment of C2C12 cells with sialidase or O-sialoglycoprotease, two enzymes which destroy functional epitopes on CD164, also inhibits differentiation. Taken together, we hypothesize that CD164 is a likely effector of the need of cell-cell contact in myogenesis and that carbohydrate-based cell recognition may be involved in this process. The Specific Aims are: 1) to identify structural determinants of the pro-myogenic activity of CD164; 2) to identify counter-receptors for CD164; and 3) to investigate the role of CD164 in myogenesis in vivo. These experiments will provide information critical to molecular and genetic understanding of CD164's role in myogenesis. This research should therefore shed light on fundamental processes by which skeletal muscles develop. [unreadable] [unreadable]